Antibody to bacterial capsular polysaccharides are the major form of protection against infections from pyogenic bacteria. The delay in ontogeny of human antibody responses to polysaccharides predisposes the healthy infant to infection and defective antibody responses renders patients with immunodeficiency, including the aged and patients with AIDS or malnutrition, at high risk. The overall goal of this proposal is to define the basis for the delayed ontogeny of human antibody responses to polysaccharides. We have found that the dominant Vk gene encoding high affinity human antibody to the Haemophilus influenzae b capsular polysaccharides is the A2 gene, which maps to the Jk-distal region of the locus, is used preferentially in germline form, and encodes antibody when expressed with an arginine at the Vk-Jk junction that arises by N region addition. We hypothesize that delayed rearrangement of critical Vk genes that encode these highly restricted antibody repertoires and delayed and infrequent N region addition contribute to this delay in ontogeny. This proposal will address whether: 1) there is a delay in a) ontogeny of immunoglobulin gene rearrangement of Vk genes located in the Jk-distal region of the human Vk locus, and b) N region addition at the Vk-Jk junction, which appears critical for the generation of an extended CDR3 loop for binding of serum antibody to polysaccharides; 2) rearrangements of Jk-distal Vk genes occur preferentially as secondary Vk-Jk rearrangements; 3) N region addition at Vk-Jk junctions occurs preferentially when L chain rearrangement occurs at an early state of B cell development; 4) Vk gene expression is affected by Ck light chain polymorphism; 5) pneumococcal polysaccharides that induce antibody responses late in infancy are encoded b map to the Jk-distal-region and require N region addition at their Vk-Jk junction; 6) deletion of the N region-derived junctional CDR3 residue of antibody to polysaccharides abolishes antibody binding. The results should provide novel insights into the ontogeny of human antibody responses and provide a background for developing vaccines that accelerate maturation of antibody responses.